The V8 Owners Forum -

I've modified my cooling system and am bleeding it. I'm nearly there now. Perhaps I am there...

It behaves as expected, though when it gets to around 85 degrees (which is around the time the stat opens) it rockets up to 115 degrees, UNTIL, I blip the throttle, then it dives straight back down to where it came from. I'm assuming this is an air bubble on the temp sensor that's getting moved when I blip the tthrottle and hence speed up the water flow?

I'm guessing this will auto bleed once I take her out for a proper spin? (I've only revved to 2k in the drive way - awaiting the return of my diff)

Never seen that, but your explanation seems plausible. Can you measure temp elsewhere to confirm it is a real effect?

Chris.

If you have a pocket of air at the temp sensor (ie. it's in the dry so to speak) then the sensor will read low not high. My guess is either air behind the stat or a sticking stat.

Regards Denis

I believe what happens is this:

Air pockets get dragged around the cooling system randomly according to the speed of water flow. ie related to engine rpm.

When one of the pockets of air pass by the exhaust manifold water galleries (by far the hottest locality) the coolant thereabouts turns to superheated steam - even at the full system pressure of approx 15 psi.

The reason it super-heats so readily is that the molecules of water have no neighbouring molecules to buffer their heat so they have to change state into steam.

With normal antifreeze solution and normal system pressure the coolant boiling points are as follows:

In relation to coolant expansion creating increased pressure in the system, the boiling point of water is raised (typically) 1.7°C for every 1 psi, so at 15 psi the boiling point will be 125°C. Addition of (say) ethylene/glycol antifreeze further raises the boiling point depending upon the mixture ratio. Typically for a 40% mix, the boiling point is raised to 135°C at 15 psi.

That is exceedingly hot and any steam generated further increases the pressure, the pressure cap will release some of it along with expelled coolant, the pressure drops the steam condenses back to fluid but not before some steam finds its way swiftly upwards towards the temperature gauge sensor and the temperature on the gauge rockets.

Shortly thereafter the system stabilises and the temperature reduces - until the next time a pocket of trapped air contacts the exhaust manifold water galleries and the process repeats - ad nauseum.

Regarding cooling system stability, something to read, here:

http://www.vintagemodelairplane.com/pag ... ing01.html

Get rid of the trapped air and the symptoms described cease.

Thanks. Most interesting.

I removed the stat and the problem vanished. I removed it because it was also opening late, and by the time it did, the fan switch didn't have time to catch up and didn't kick in until 100.

Now its coming in at 90 and I've gained a few degrees.

Next up is pulley sizes...

RogerD wrote:Thanks. Most interesting.

I removed the stat and the problem vanished. I removed it because it was also opening late, and by the time it did, the fan switch didn't have time to catch up and didn't kick in until 100.

Now its coming in at 90 and I've gained a few degrees.

You'll probably be coming back to the cooling system in the future.

Here is some more interesting stuff.

http://www.vintagemodelairplane.com/pag ... emp01.html

http://www.vintagemodelairplane.com/pag ... ant01.html

Don't overlook the possibility that the super-heating symptom may have been cured by not having any trapped air after refilling, and not due to omitting the thermostat!

Nice one - some good reading!

Have you drilled a hole in your stat?

I did, but I ended up removing the stat and it behaves much better now.

If this is the same engine which has the undersized crank pulley then you need to sort that first before you wonder why the engine gets hot at low revs. The problem may just be lack of flow through the engine.

Regards Denis

Yeah I know, but I had to replumb it anyhow as I had a faulty kenlowe stat which I replaced with an inline jobby so I took the opportunity to tidy up the piping whilst the fluid was out.

The crank pulley I'll deal with over winter when its not inconvenient to remove it and take it to my local fab, or, use it to compare to others at a breakers.